BUD-03 defines how a user publishes their preferred Blossom servers, so clients know where to upload blobs and where to look when a media URL stops working.

How It Works

Users publish a replaceable kind 10063 event with one or more server tags. Each tag contains a full Blossom server URL.

Clients can then:

  • upload blobs to the user’s preferred servers
  • discover likely blob locations from the author’s pubkey
  • retry retrieval from the listed servers when an older URL breaks

Reader-Useful Details

The order of server tags matters. The spec says users should list their most trusted or reliable servers first, and clients must at least try the first server for uploads. That means BUD-03 is not just a directory, it is also a weak preference signal.

The retrieval guidance is also practical: when a client extracts a blob hash from a URL, it should use the last 64-character hex string in the path. This helps clients recover blobs from both standard Blossom URLs and non-standard CDN-style URLs that still embed the hash.

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